In such hand-guided coordinate measuring apparatus, the probe can be guided more or less freely in space or in a plane by the operator in dependence upon the number and position of the rotational or pivot axes and can be placed on the workpiece to be measured. Coordinate measuring apparatus of this kind are described, for example, in U.S. Pat. Nos. 5,402,582 and 5,396,712.
In coordinate measuring apparatus of this type, the spacings of the rotational axes from each other must be precisely known because these spacings and the rotational angles, which are supplied by encoders mounted on the rotational axes, determine the position of the probe in space. The same applies to the spacing of the last rotational axis to the position of the probe or to the position of the axis of the probe pin to which the probe ball is attached.
Temperature has a great influence on the measuring accuracy of these apparatus because the spacing of the pivot axes can change as a consequence of the thermal longitudinal expansion of the arms connecting the joints. For this reason, the apparatus disclosed in U.S. Pat. No. 5,402,582, which is primarily made of aluminum, is provided with temperature sensors in order to detect the thermal longitudinal expansion via a temperature measurement. The position of the probe ball in space can then be corrected via a corresponding correction computation while considering the signals supplied by the temperature sensors.
This solution, however, is complex and subjected to disturbances because of the required additional sensors and computation operations.
With respect to the apparatus described in U.S. Pat. 5,396,712, it is noted that the movable parts can, for example, be made of carbon-fiber-reinforced plastic (CFK) which combines a very low mass at high stability of the assembly with simultaneous insensitivity with respect to thermal effects. However, CFK has no negligible thermal expansion coefficient. Rather, this expansion coefficient is anisotropic, that is, it is dependent upon the alignment of the fibers or, more specifically, how the particular component is wound or cut with respect to the fibers. In dependence thereon, the thermal expansion coefficient of CFK lies between -3.multidot.10.sup.-6 /K and approximately +23.multidot.10.sup.-6 /K which for lever lengths of one meter means length changes of between -30 and +230 .mu.m in a temperature interval of 10.degree.. This amount is significant with respect to measurement uncertainty when high requirements are imposed on measuring precision.